Touch sensors detect touch, such as from a user's finger, a stylus, or some other device. Touch sensors may be transparent or opaque input devices for computers or other electronic systems. A transparent touch sensor, such as a touch screen, may be used in conjunction with a display device. Touch displays are increasingly used in commercial and consumer applications, such as cellular phones, restaurant order entry systems, industrial process control applications, interactive museum exhibits, public information kiosks, pagers, computers, personal digital assistants, and video games.
Acoustic-based sensors may be used to detect touch. Certain types of acoustic touch screens, also known as ultrasonic touch screens, detect touch with high transparency and high resolution, while providing a durable touch surface such as a glass touch substrate. Of particular commercial interest are ultrasonic touch screens using surface acoustic waves.
A border, bezel, or other edging may be used to block a user's view of electronics and components within a touch sensor device. For a transparent touch screen, paint may be used on a back side of the touch substrate as the border. For acoustic touch sensor devices having electronics and components on the back side of the touch substrate, the border paint may undesirably attenuate the surface acoustic waves. Thicker paint on the border area of the touch substrate tends to more greatly attenuate the surface acoustic waves, resulting in decreased signal-to-noise ratio and more likely incorrect touch detection. Thicker paint also decreases the surface acoustic wave velocity, which may not be desirable. Some paint colors may be more opaque than others, allowing for thinner paint layers. White paint is not such a paint color, and obtaining optical density greater than about 1.0 for white paint is difficult without using a thick layer. To sufficiently block light, a thick layer of white paint (or multiple coats of white paint) is needed in contrast with the thickness needed with black paint. As mentioned, the thicker paint border layer is undesired. However, a certain level of opacity is sometimes necessary, for example especially for applications that are backlit.